Sounds like you need to have an plethora of answers:) and i will give you a few pointers without going overboard or I will be writing this all knight :) As far as sites I am a member of and highly recommend joining Bonsainut.com, where you will be able to look through old and active posts to learn as well as talk to the community and question them. As far as good beginner's learning resource I recommend going to your local library as they should have a few dozen.

As for your questions here we go:) I dont have too much experience with Gardenias myself but I did just repot about 6 I acquired through cuttings. First off it is important that the soil is fee draining. This means that the soil mixture is not simply your run of the mill potting soil. It should be able to dry sufficiently (wet no longer than 3 days) as well as hold water sufficiently. A little confusing for a beginner I know but it will make more sense as you learn.

As a beginner one of the main reason a Bonsai dies is from over watering. the tree should not be watered every day and should be allowed to dry sufficiently before watering. You should stick your finger in the soil up to around the first knuckle, when soil feel almost completely dry with just the slightest tinge of moisture its time to water again (never allow it to completely dry out). A method I teach beginners is the chop stick method. acquire a cheap chopstick like they offer at a Chinese restaurant. stick the chopstick in the soil twords the edge of the pot. Check it daily and when the stick is dry its time to water. Teach yourself what the soil feels like when its time to water and after awhile you will no longer need the chopstick.

As for fertilizer, the point is to give the tree just enough so it gets the nutrients it needs and no more. Over fertilizing will make the growth leggy and sparse which is the opposite of what you are trying to accomplish for bonsai. Miracid is made by Miracle Gro and can be found at any hardware store or nursery. That being said i would not recommend using it for Bonsai, especially if you are a beginner as Miracid Will burn the roots easily, killing the tree. I would recommend using a fertilizer with and equal NPK ratio similar to 10-10-10. Also be sure to ask for one that contains micro &macro nutrients or other trace elements in it as well. While Gardenias are acid loving plants similar to Azaleas you do not need to use a Acidic fertilizer, just be sure it is one that will not effect the trees current soil by raising its alkalinity. Also be sure to never use more than the recommended dosage, in fact to be safe it is better to use less than the recommended dosage.

When it comes to a fertilizing regiments I cant really give you a good plan as I dont know what stage in training your trees are and what you are looking to accomplish. That being said if I had to give some sort of time frame, I would recommend fertilizing once every other week during the summer with a general purpose NPK rated fertilizer. And then during the winter do so once a month with a low Nitrogen NPK rating similar to 2-10-10. this is due to the fact that nitrogen plays a large role in growth of foliage and your plant will not need it during the winter as any strong growth will be undesirably formed. The tree is still growing though (unlike deciduous trees) during the winter and therefore should still receive a small amount of nitrogen to keep it happy.

This last part is assuming you live in an are with a winter since I have no clue where you are located. If you have any more follow up questions you can message me on Bonsainut.com where I have the same user name. I dont come to this site very often any more.

Hope this helps clarify some things for you.
-Mike

Hi kokoro,
Never feed your gardenia bonsai when in flower. Remember this: your plant needs food when is growing a lot, and it hapens during spring and summer.
Begin to feed every two weeks, beginning in the first spring week. And stop at the last summer week.
Hope was usefull.
cheers

Here is a link that might be useful: Gardenia Bonsai